You can put commands beginning with / in the green text block and nest it inside a red command block, Commands like /summon zombie for example. Only, it's not necessary to put the first slash. If a command has more than one / then you can put the 2nd / in the green text block.
You should be able to use most simple vanilla commands like that, but not command blocks, console commands or anything more complex.
How to Use the Player Chat Functionality
- write your mod.
- click the "execute" icon (green triangle in lower left corner of screen)
- press keyboard "m" to run the command when prompted.
- press keyboard "Esc" to get your code back on the screen, code is still running, though there is no indication of this.
- press keyboard "/" (slash) to get a text entry prompt at the bottom of the screen.
- press keyboard "Enter" to submit your written text, Chat or command to your program!